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Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock()

From Peter Zijlstra <peterz@infradead.org>
Newsgroups linux.kernel
Subject Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock()
Date 2015-07-14 10:40 +0200
Message-ID <pM3US-8kr-13@gated-at.bofh.it> (permalink)
References (5 earlier) <pLQEi-fA-13@gated-at.bofh.it> <pLRTH-UK-11@gated-at.bofh.it> <pLSmK-1jC-29@gated-at.bofh.it> <pLUeS-2u1-13@gated-at.bofh.it> <pLUHU-2DL-11@gated-at.bofh.it>
Organization linux.* mail to news gateway

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On Tue, Jul 14, 2015 at 08:43:44AM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2015-07-14 at 00:15 +0200, Peter Zijlstra wrote:
> 
> > > This is instead the sequence that is of concern:
> > > 
> > > 	store a
> > > 	unlock M
> > > 	lock N
> > > 	load b
> > 
> > So its late and that table didn't parse, but that should be ordered too.
> > The load of b should not be able to escape the lock N.
> > 
> > If only because LWSYNC is a valid RMB and any LOCK implementation must
> > load the lock state to observe it unlocked.
> 
> What happens is that the load passes the store conditional, though it
> doesn't pass the load with reserve. However, both store A and unlock M
> being just stores with an lwsync, can pass a load, so they can pass the
> load with reserve. And thus inside the LL/SC loop, our store A has
> passed our load B.

Ah cute.. Thanks, clearly I wasn't awake enough anymore :-)
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[RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-13 14:20 +0200
  Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Peter Zijlstra <peterz@infradead.org> - 2015-07-14 10:40 +0200
  Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-14 12:10 +0200
    Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-14 21:40 +0200
      Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-15 03:40 +0200
        Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-15 13:00 +0200
          Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-15 15:20 +0200
  Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Michael Ellerman <mpe@ellerman.id.au> - 2015-07-15 05:10 +0200
    Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-15 12:50 +0200
      Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Michael Ellerman <mpe@ellerman.id.au> - 2015-07-16 04:10 +0200
        Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-16 17:20 +0200
          Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-07-17 01:00 +0200
            Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-17 11:40 +0200
              Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Peter Zijlstra <peterz@infradead.org> - 2015-07-17 12:20 +0200
                Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-17 14:50 +0200
              Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-07-18 00:20 +0200
                Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-20 15:40 +0200
                Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-20 15:50 +0200
                Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-07-20 16:00 +0200
    Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-15 16:20 +0200
      Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Michael Ellerman <mpe@ellerman.id.au> - 2015-07-16 03:40 +0200

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